Kevin Jones wrote:
> It's probably not a bottleneck but could be a factor,
> sometimes large. Conversions themselves are just overhead
> that should be avoided, if at all possible, where I come
> from.
That itself sounds like folklore. The fact is XQuery can be a very
computationally expensive operation. A large reason why this language
was invented was to enable native XML databases that can create their
own indexable, optimizable data structures that are more amenable to
querying than traditional models like DOM and XOM. Some engines with
some queries show two or three orders of magnitude improvement by using
their internal models instead of directly querying DOMs or XML files.
Of course some queries are so fast anyway that improving by two orders
of magnitude means you go from 10ms to a tenth of a millisecond, and if
you only need to do that once, who cares? But sometimes the speed gain
does matter.
Queries can easily be O(N^2) or worse, depending on what they're asking.
By contrast a conversion to a native model should normally be able to be
done in O(N), where is N is the size of the document. If the conversion
includes building an index that reduces ultimate query time to O(NlogN)
or less, conversion may be worth it for large documents and complicated
queries. The cost you worry about is normally the size of the additional
model you have to build. Operating directly on the original data
structure, whether through its own native interface or an LCD interface,
saves the memory costs of building an additional object structure to
hold the same content. Whether speed or size matters more varies from
one problem and environment to the next.
Using an LCD interface doesn't really affect this calculation one way or
the other. The engine can always choose whether to create a more
efficient internal model tailored to its needs or operate directly on
the native data structure depending on whether it cares more about size
or speed (and on how efficient its internal representation is). It can
make this choice both with native and LCD interfaces. The only
difference is whether there have to be N*M converters (where N is the
number of engines and M the number of models) or only M LCD interfaces.
--
Elliotte Rusty Harold elharo@metalab.unc.edu
XML in a Nutshell 3rd Edition Just Published!
http://www.cafeconleche.org/books/xian3/http://www.amazon.com/exec/obidos/ISBN=0596007647/cafeaulaitA/ref=nosim